1. Field of the Invention
The present invention relates to a die structure for injection molding of a light alloy free from casting defects, and method for injection molding using the same.
2. Prior Art
Light alloys containing of a matrix of aluminum or magnesium, particularly magnesium based alloys containing aluminum as an alloy component, have attracted special interest recently as materials, which are of light-weight and capable of securing a predetermined mechanical strength by means of plastic working such as forging. However, these light alloys show greatly thermal shrinkage during casting or molding, and this allows the fluidity to be lowered unless the casting temperature is raised in the gravity casting. Consequently, any perfect, sound cast free of cavity defect is not obtained. However, the high casting temperature of the melt can show the coarse-grained microstructure in the cast alloy because of low cooling rate in the cooling step of the casting process, then resulting in the reduce in workabilty of the material.
On the other hand, a desirably fine-grained structure can be obtained by die casting the alloy. In this process, since the molten metal is injected at a high pressure in a spraying state into a cavity of the mold, a great number of small voids or pores are left in the die cast due to a contained gas, and reduce mechanical strength of the cast so that any cast material having high properties can not be obtained. Particularly, for a thick-walled part, the strength is drastically lowered in this die casting process.
An object of the present invention is to provide a mold structure for injection molding a molten light alloy, capable of producing it with a fine-grained structure free from gas defects, then improving mechanical property of the light alloy cast material.
Another object of the present invention is to provide a method for injection molding a molten light alloy capable of producing it with a fine structure free from gas defects, then improving mechanical property of the light alloy cast material, then improve mechanical property of the light alloy cast.
The present invention provide a mold for injecting and a method for obtaining fine-grained microstructure free from casting defects such as blow holes or shrinkage voids in the alloy during injection molding.
In the invention, the molten metal is injected into the internal cavity of the die in a laminar flow state in the injection molding method, a fine structure free from gas defects can be obtained.
The present invention provides a mold structure for injection molding into an interior cavity portion through a gate a light molten alloy which is in a semi-molten state where a solid phase and a liquid phase of the alloy coexist or in a full molten state remaining at a temperature just above the liquidus point of the alloy, wherein a ratio S1/S2 of a sectional area S1 of the gate with respect to a maximum sectional area S2 of the internal cavity perpendicular to the molten metal flowing direction is set to be not less than 0.06.
According to the present invention, by setting the gate sectional area larger than such special value to the maximum sectional area of the internal cavity portion in the direction perpendicular to the metal flowing, or poured, direction toward the cavity, the molten alloy can become in the laminar flow state in the cavity. As a result, no generation of such gas defects as blow holes or shrinkage voids is substantially observed in the injection-molded product produced.
For the injecting mold of the invention the lower limit of the areal ratio S1/S2 should be 0.06. As the areal ratio S1/S2 is less than 0.06, as shown in FIG. 3, the relative density of the product is drastically lowered because the generation rate of such gas defects increases.
On the other hand, the upper limit of the areal ratio S1/S2 of the mold preferably may be 0.50. As the ratio S1/S2 is more than 0.5, the relative density of the molded material would be on almost the same level as that of the conventional die cast, causing an advantage of using such semi-melt injection molding method to disappear.
In the case where a thick-walled product is molded, in the melt filled in the corresponding thick portion of the cavity is apt to be finally solidified to produce shrinkage cavities or voids in the portion. In this case, it is preferred to insert core pins into the internal cavity portion of the mold, and then, in use, to pressurize the molten metal by push the core pins inward the cavity immediately after pouring, thereby to prevent shrinkage cavities from occurring during solidification. Thus the core pins cause the semi-molten alloy which is solidifying to flow plastically, resulting in crushing of the shrinkage cavities in the product.
However in this case of the thick-walled product, as a solid fraction (a volume fraction of the solid phase in the semi-molten melt) is low in the melt, the gas defects tends to be formed in the alloy product. The solid fraction lower than 10% causes both the relative density and tensile strength to be rapidly lowered as shown in FIGS. 7 and 8. Accordingly, for production of the thick-walled product, the semi-melt injection molding is preferably performed at the solid fraction which may be prepared to be not less than 10%.
With the decrease of the solid fraction, the average solid grain size is liable to become small and the creep characteristics at high temperature are liable to be lowered as shown in FIG. 6. To secure the predetermined creep characteristics, injection molding must be performed under the condition that not only the solid fraction is not less than 5%, but also the average crystal grain size in the solid phase contained in the melt is not less than 50 xcexcm.
The relative density of the injection-molded material of the present invention can be improved by optionally pressed or forged. The draft (a ratio of difference of the an initial thickness and the deformed thickness of the material with respect to the initial thickness) due to pressing or forging should be set to not less than 25%. The reason is that the relative density, as shown in FIG. 4, is rapidly increased from the draft of 20% and is saturated at 25%.
The method of the present invention is preferably applied to magnesium based alloy containing 4 to 9.5% by weight of aluminum as a main alloying component, as the light alloy. When the aluminum content is smaller than 4% by weight, an enhancement in mechanical strength is not expected. On the other hand, when the content exceeding 9.5% by weight can significantly lower workability (by limiting upsetting rate).
The light alloy obtained by the present method is preferably subjected to heat treatment for Temper T6 (composed of a solution treating followed by an artificial aging) for further improving the mechanical strength.
Thus, the present invention can provide the molded material of a light alloy free from gas defects by injection molding process, so that such molded material, even if it may have a rough shape, can be forged into a final product having excellent mechanical strength and precise dimensions.